In recent years, as information is delivered more and more through multimedia, it is becoming a common practice in cellular mobile communication systems to transmit not only audio data but also large-volume data such as still image data and video data. To realize transmission of large-volume data, technologies to achieve a high transmission rate using a high-frequency radio band are being studied extensively.
However, when a high-frequency radio band is used, a high transmission rate can be expected in short distance transmission, whereas attenuation caused by the transmission distance grows as the distance increases. Therefore, when a mobile communication system using a high-frequency radio band is actually operated, since a coverage area of a radio communication base station apparatus (hereinafter abbreviated as “base station”) decreases, more base stations need to be installed. Installation of such base stations requires a considerable amount of cost. Therefore, there is a strong demand for a technique for realizing a communication service using a high-frequency radio band while suppressing increases in the number of base stations.
To expand a coverage area of each base station in response to such a demand, a relay transmission technique is under study which installs a radio communication relay station apparatus (hereinafter, abbreviated as “relay station”) between a base station and a radio communication mobile station apparatus (hereinafter, abbreviated as “mobile station”) and performs communication between the base station and the mobile station via the relay station. Using the relay technique, even a mobile station unable to directly communicate with the base station can realize communication via the relay station.
LTE-A (Long Term Evolution Advanced) systems seeking to introduce the above-described relay technique are required to maintain compatibility with LTE (Long Term Evolution) from the standpoint of the smooth transition from LTE and the coexistence with LTE. Therefore, regarding the relay technique, LTE-A is also required to maintain interchangeability with LTE. Studies are being carried out on the possibility in LTE-A systems that MBSFN (MBMS Single Frequency Network) subframes may be set on a downlink (hereinafter referred to as “DL”) during transmission from a base station to a relay station to achieve interchangeability with LTE.
Here, communication between the base station and mobile station is carried out via the relay station through a time division relay (that is, TD relay). FIG. 1 is a diagram illustrating the TD relay. FIG. 1A is a conceptual diagram illustrating a TD relay on a downlink and FIG. 1B is a conceptual diagram illustrating a TD relay on an uplink. In the TD relay (also referred to as “half duplex relay” or “type 1 relay”), transmission from the base station to the relay station and transmission from the relay station to the mobile station are divided by time.
On the uplink, as shown in FIG. 1B, transmission from the mobile station to the relay station is performed using an access link in subframe #2 and communication from the relay station to the base station is performed using a backhaul link in subframe #3. In subframe #4, transmission from the mobile station to the relay station is performed again.
Similarly, on the downlink as shown in FIG. 1A, transmission from the relay station to the mobile station is performed using an access link in subframe #2 and communication from the base station to the relay station is performed using a backhaul link in subframe #3. In subframe #4, transmission from the relay station to the mobile station is performed again.
As described above, by dividing communication between communication using the backhaul and communication using the access link of the relay on the time axis, it is possible to divide time between a transmission time and a reception time for the relay station. Therefore, the relay station can perform a relay without being affected by a wraparound between transmitting and receiving antennas.
Furthermore, on the downlink, an MBSFN subframe is set for the access link. The “MBSFN subframe” is a subframe defined to transmit MEMS (Multimedia Broadcast Multicast Service) data. For LTE terminals, operation is defined which stipulates that reference signals should not be used in the MBSFN subframe.
Thus, LTE-A proposes a technique that sets a subframe for an access link which overlaps with a subframe for a backhaul link used for the relay station to communicate with the base station in the MBSFN subframe. This proposal prevents the LTE terminal from erroneously detecting reference signals.
FIG. 2 shows an example of allocation situations of control signals and data for the respective stations of the base station, relay station and mobile station when using subframes of an LTE system. As shown in FIG. 2, in the LTE system, a downlink control signal transmitted or received at each station is arranged in a PDCCH (Physical Downlink Control Channel) region of the head of each subframe. That is, both the base station and the relay station transmit a control signal using the PDCCH region of the head of each subframe. Focusing on the relay station, since the relay station has to transmit a downlink control signal to the mobile station even in an MBSFN subframe, the relay station transmits the control signal to the mobile station, then switches to reception processing and thereby prepares for reception of a signal transmitted from the base station. However, since the base station also transmits a downlink control signal directed to the relay station at timing at which the relay station transmits the downlink control signal to the mobile station, the relay station cannot receive the downlink control signal transmitted from the base station. In order to avoid such inconvenience, in LTE-A, studies are being carried out on the possibility of providing a region (R-PDCCH (relay PDCCH) region) for arranging a downlink control signal for the relay station in the data region.
In LTE, PDCCH includes a DL grant indicating DL data allocation and a DL grant indicating UL data allocation. In LTE-A, studies are also being carried out on the possibility of including a DL grant and DL grant in R-PDCCH. Furthermore, studies are also being carried out on the possibility of arranging a DL grant in a 1st slot and arranging a UL grant in a 2nd slot in R-PDCCH (see Non-Patent Literatures 1 and 2). Thus, by arranging the DL grant only in the 1st slot, it is possible to shorten a decoding delay of the DL grant and prepare for transmission of ACR/NACK for DL data (transmitted 4 subframes after reception of a DL grant in FDD). Furthermore, as shown in FIG. 3, studies are also being carried out on the possibility of making a resource block (FRB) in a physical layer where the R-PDCCH region is provided differ from one relay station to another. Thus, the relay station performs blind decoding on a downlink control signal transmitted from the base station using the R-PDCCH region within a resource region indicated by higher layer signaling from the base station and thereby finds a downlink control signal directed to the relay station.
Next, a method of allocating a downlink data channel (PDSCH) will be described. There are three types of PDSCH allocation methods; type0, type1 and type2. In type0 allocation PDSCH is allocated in RBG (Resource Block Group) units. In type1 allocation, PDSCH is allocated in RB units. In type2 allocation, continuous RBs are allocated, and a start RB and an end RB are reported. Here, RBG is a unit that groups a plurality of RBs. In LTE, the size of RBG is defined according to the number of RBs within a system band (see FIG. 4).
According to Type0 allocation, since many RBs can be specified with a small number of bits, all RBs within the system band can be allocated. Furthermore, in Type1 allocation, a subset is defined per RBG. That is, “subset” is an RBG group. For this reason, in Type1 allocation, RBs included in RBG constituting an allocated subset can be allocated, whereas there are also RBs that cannot be allocated. FIG. 5 shows a specific example. In FIG. 5, it is assumed that the RBG size is 3 RBs and an allocation bit sequence is “1, 0, 1, 1, 0, 0.” According to this allocation bit sequence, it is shown that in type0 allocation, allocations exist in RBGs#0, #2 and #3. That is, RB#0, RB#1 and RB#2 included in RBG#0, RB#6, RB#7 and RB#8 included in RBG#2, and RIM, RB#10 and RB#11 included in RBG#3 are allocated. On the other hand, in Type1 allocation, information as to which subset is allocated is further necessary. Here, suppose subset#0 is allocated. In FIG. 5, RBG#0 and RBG#3 are included in subset#0. Therefore, according to the above-described allocation bit sequence, RB#0, RB#2 and RB#9 to which bit “1” is allocated are allocated from among RB#0, RB#1, RB#2, RB#9, RB#10 and RB#11 included in RBG#0 and RBG#3. Thus, in type1, allocation, only RBGs included in subsets can be allocated, and therefore all RBs included in the system band cannot be allocated.
Furthermore, LTE-A is studying two methods; a method that transmits a plurality of downlink control signals directed to a plurality of relay stations by interleaving them before allocating them to RBs in an R-PDCCH region and a method that transmits the downlink control signals without interleaving.
In the case where downlink control signals are interleaved and then transmitted, a plurality of R-PDCCH regions are shared among a plurality of downlink control signals, and therefore there is a feature that the number of RBs in which the respective downlink control signals are arranged increases. When the number of RBs in which downlink control signals are arranged increases, it is possible to obtain diversity gain more easily. Furthermore, in the case where downlink control signals are interleaved and then transmitted, the number of RBs is set in which downlink control signals directed to a plurality of relay stations and to be interleaved together (hereinafter referred to as “interleaving group”) are arranged (that is, the number of R-PDCCH regions). The number of RBs in which this interleaving group is arranged is called “virtual band width.”
In the case where downlink control signals are transmitted without interleaving, only a downlink control signal directed to one relay station is included in one RB. Therefore, since the number of RBs in which downlink control signals directed to one relay station are arranged decreases, there is a feature that it is more difficult to obtain diversity gain. However, since an RB of good channel quality can be allocated to each relay station, it is possible to obtain scheduling gain.
LTE-A is studying a method of allocating a downlink data signal directed to a relay station (R-PDSCH (relay PDSCH) signal) to an RBG to which the downlink control signal directed to the relay station is allocated (e.g., see Non-Patent Literature 1). FIG. 6 shows an example of a method of interleaving and transmitting downlink control signals and a case where a Type0 allocation method is adopted. When an allocation bit corresponding to a target RBG is “1,” a 2nd slot portion of RB with a DL grant allocated (portion indicated by (b) in FIG. 6) and RB with no DL grant allocated (portion indicated by (c) in FIG. 6) are allocated as regions (R-PDSCH regions) in which downlink data signals are arranged. On the other hand, when the allocation bit corresponding to a target RBG is “0,” no region in which a downlink data signal is arranged is allocated to the RBG.